1. Field of the Invention
The present invention relates to a sheet heater for use in a seat and the like, and more particularly to a sheet heater incorporated in a seat for a vehicle.
2. Background art
A sheet heater incorporated in the seat is used for warming up a seat for a vehicle. FIG. 8 is a perspective view for illustrating an example of a conventional sheet heater incorporated in a seat for a vehicle. With reference to FIG. 8, sheet heater 101 is incorporated between external surface 103 and main pad 104 of seat 102.
In recent years, it has been requested to add means of cooling a seat itself as one of means of further improving passenger comfort in the environment of the vehicle interior. Seat cooling system using Peltier devices in combination has been put to practical use. However, with this technique, because warm air is blown out of the surface of the seat at heating, the temperature feeling is lower than the actual temperature of the warm air and a sufficient heating effect cannot be obtained. As a countermeasure against this phenomenon, a combination of a conventional sheet heater and cooling using Peltier devices or air blasting using a fan is considered. As a factor necessary for this countermeasure, improvements in air-passing capability of the sheet heater are requested.
An example of an air-passing sheet heater is described in the Japanese Patent Unexamined Publication No. H08-507404. FIG. 9 is a sectional view of a conventional sheet heater described in this publication.
With reference to FIG. 9, a heating element (hereinafter referred to as a “sheet heater”) is made up of electrically-insulating and air-passing carrier layer (hereinafter referred to as “base material”) 105, heating loop (hereinafter referred to as “heating wire”) 106 formed on carrier layer 105, and friction layer (hereinafter referred to as “protective material”) 107 for protecting the heating wire. Examples of the air-passing base material include a base material having a mesh structure. Conventionally, adhesive has been used to fix heating wire 106 to base material 105 and fix protective material 107 to base material 105 and heating wire 106.
However, the conventional structure has a problem: thermal stress caused by repeated electrical conduction deteriorates the adhesive that fixes heating wire 106, and heating wire 106 is peeled from base material 105 by the load imposed on the seat during sitting.
There is another problem: because heating wire 106 is fixed to base material 105 using adhesive, fixed heating wire 106 has an extremely low degree of freedom and thus heating wire 106 is likely to be broken by the load imposed thereon. In other words, because heating wire 106 is fixed to base material 105, when wrinkles are generated by the load, such as weight imposed on the seat by sitting, heating wire 106 follows the wrinkles in base material 105 and bends. As a result, the load is concentrated on heating wire 106 in these wrinkles, and repeated bending leads heating wire 106 to breakage. Especially when through-holes are provided in base material 105 to impart air-passing capability, decrease in the strength of base material 105 is likely to generate wrinkles in base material 105. As a result, heating wire 106 fixed to a base material having through-holes is more likely to be broken than the case without the through-holes.
Further, because heating wire 106 is fixed to base material 105 using adhesive, base material 105 is impregnated with the adhesive and base material 105 itself is hardened. This poses another problem of reducing user's comfort of sitting in the seat.
Additionally, when base material 105 having a mesh structure is used to improve air-passing capability, the area in which heating wire 106 is in contact with base material 105 is smaller. Therefore, in order to sufficiently hold heating wire 106 against the weight applied to the seat during sitting, it is necessary to fasten heating wire 106 between base material 105 and protective material 107. This arrangement can improve the adhesive strength toward heating wire 106 and reduce the weight directly applied to heating wire 106. However, this arrangement poses a problem of increasing the number of members and operations in production.